Heat-developable photographic materials are known, and the materials as well as methods of processing the same are described in, for example, Shashin Kogaku no Kiso (Bases of Photoengineering), Volume of Non-silver Photography, pages 242 to 255 (published by Corona Co., Ltd., 1982) and U.S. Pat. No. 4,500,626.
Numerous methods have been proposed for formation of positive color images by heat-development.
For instance, U.S. Pat. No. 4,559,290 has proposed a method of using a so-called DRR compound in the form of an oxidized compound having no dye-releasing ability together with reducing agent, in which the reducing agent is oxidized in accordance with the exposure of the silver halide in a photographic material by heat-development and the DRR compound is reduced by the remaining reducing agent which has not been oxidized to release a diffusible dye. European Patent 220,746A and Disclosure Bulletin 87-6199 (Disclosure Bulletin, Vol. 12, No. 22) mention a method of forming a positive image by heat-development using a novel compound which may release a diffusible dye by a similar mechanism.
In the positive image-forming method mentioned above where a reducible dye-forming compound is used, in general, an electron-donating agent and an electron-transferring agent are used together as a reducing composition. However, the use of such a combination has been found problematic in that the electron-transferring agent will gradually decompose to be lost during storage. As a result of the decomposition and loss of the electron-transferring agent during storage, the image to be obtained is increasingly stained. In order to overcome the problem, a means of using an electron-transferring agent precursor which is stable during storage is helpful. However, the use of such electron-transferring agent precursor has a drawback in that a color image with sufficient S/N ratio can not be obtained.